476. Now the globules of vapor, of which fogs and clouds are composed, are arranged in a similar manner throughout the atmosphere, and act upon the light of the sun and moon as if they were so many small glass balls. When, therefore, the rays of the moon, for instance, reach the eye of the observer, after passing between the particles of light, interposing vapors, he will often see her orb surrounded by beautiful coronas, glowing with the rich colors of the spectrum.

477. Coronas are only seen when the globules of vapor are comparatively few, and are of equal size. If they are too numerous a dense cloud is formed, and the intervals being closed by the globules, no rays can pass through them. If they are few in number but differ in size, then the intervals are not symmetrically arranged, and the sun or moon will appear surrounded by a glory, or bright circle of white light.

The distance of coronas from the luminous body is not always the same. The smaller the particles, the greater is the diameter of the rings.

478. When white clouds, having the form of the cirro-cumulus, float near the sun, bright, prismatic colors are often seen, by the aid of a blackened mirror,

When are coronas only seen?

What is the result when the particles of vapor differ in size?

Why will the rings vary in magnitude?

What is said respecting the edges of cirro-cumulus and cumulus clouds, when passing near the sun and moon?

fringing the edges that are parallel to the horizon. 'The fringes are generally green within, bordered by two red lines.

If the air is pure, and the moon shines brightly, the light and broken edges of cumulus clouds, as they pass near her disk, are sometimes seen in like manner fringed with prismatic hues, the purple tint being the nearest color, and the red the most distant.

479. ANTHELIA. Anthelia are coronas seen by reflec tion, when the back of the observer is towards the sun; and are so called from the Greek words anti, opposite, and helios, the sun.

If the plain surface of the circularly ruled glass (Art. 474) is blackened, and the luminous object seen by reflection upon the ruled side, its image will appear surrounded by colored rings precisely like those that encircle the object itself, when viewed, as in the first case, by transmitted light.

In analogy to this, when the shadow of a person is cast upon a stratum of vapor, the head of the observer, under favorable circumstances, is seen surrounded with prismatic circles.

480. A beautiful display of this kind was witnessed from the summit of Mount Lafayette, fifteen miles from Mount Washington, on the 7th of August, 1826. In the afternoon of the day in question, two gentlemen were standing upon this lofty eminence, a thunderstorm was raging beneath them, and a sea of vapor shut out the vales from view. A light mist was at this time falling, when suddenly the sun burst through the clouds above, and the observers saw their shadows resting upon the vapor before them, their heads surrounded with brilliant, prismatic rings. The circles were apparently ten or twelve feet in diameter, perfectly defined, and their tints were exceedingly rich and vivid. This phenomenon lasted for the space of twelve or fifteen minutes, when it gradually vanished.

481. In the polar seas, when the stratum of fog that

Define anthelia. Relate the instances given.

rests upon the ocean rises to the height of about three hundred feet, a person, stationed upon the mast of a ship, eighty or a hundred feet above the water, perceives in the fog opposite the sun, one or more circles around the shadow of his head. They are all concentric;. their common centre being in the imaginary line drawn from the sun through the eye of the spectator to the fog beyond him. The number of circles varies from one to five, and when the sun is bright, or the fog thick and low, they are usually numerous and highly colored.

482. On the 23d of July, 1821, Scoresby saw four concentric circles around his head, with the series of colors arranged in the following order:

1st circle, white, yellow, red, purple.

2d circle, blue, green, yellow, red, purple.

3d circle, green, whitish, yellowish, red, purple. 4th circle, greenish white.

The colors of the first and second rings were very brilliant, those of the third faint, and only seen at intervals, while the fourth exhibited only a slight tinge of green. According to Scoresby, anthelia are always seen in the polar regions whenever fog and sunshine occur at the same time.

483. Several philosophers have supposed that anthelia, or coronas opposite to the sun, are caused by the passage of light through frozen particles of vapor, but this phenomenon has frequently occurred, when the temperature of the air was so high as to preclude this idea.

Thus Kaemtz often beheld anthelia upon the Alps, when the temperature of the air was 50° Fah., at a short distance from the fog. Their explanation upon the principle of diffraction is the most satisfactory, and the truth of this theory is strongly confirmed by an observation of Kaemtz, who, on one occasion, first saw a corona when the cloud was between himself and the sun,

What is the opinion of some philosophers in regard to anthelia?
What objection can be urged against this view?

What fact is stated by Kaemtz?

and then an anthelion from the same cloud when it was

opposite to the sun.” 66

HALOES.

484. Haloes are circles of prismatic colors about the sun and moon; they differ from coronas in three particulars; first, their structure is often more complicated; secondly, their diameter is greater; and thirdly, the order of colors is reversed, the red being nearest the luminary.

485. The several parts of this phenomenon may be thus classified, 1st, Circles surrounding the orb which occupies their centre. 2d, Circles passing through the orb. 3d, Arcs of circles touching those of the first class. 4th, Parhelia and paraselena or mock-suns and mockmoons, found at the points where the circles cross each other.

486. FACTS. The annexed figure represents a halo around the sun, observed by Scheiner, in 1630. In the cut, S is the sun, A B C à circle about 45° in diameter, and D E F another circle, its diameter being nearly 95° 20', the sun being in their Both the common centre. circles were colored like the primary rainbow, but the red was next to the sun, the other colors succeeding__in the natural order. DS Fis a third whitish circle pass

Fig. 37.

B

F

SOLAR HALO

ing through the centre of the sun, and H E G a portion of a fourth touching D E F at E. At A, C, D, and F, were mock-suns; the same phenomena were seen at B

What are haloes?

How do they differ from coronas or crowns?

How are the several parts of the halo classified?
Describe the three haloes recorded.

and E. The mock-suns, A and C, were of a purplish red next to the sun, while D and F were entirely white, the former were also more brilliant, continuing visible for three hours together, while the light of the latter was faint and fluctuating.

The mock-suns B and E were almost the first to appear and the last to fade, excepting A, and throughout the whole phenomenon, which lasted five hours, they were perpetually changing in magnitude and color. B was formed in a peculiar manner, the halo A B C was composed of several intersecting circles, and at one of these intersections the mock-sun B appeared.

487. On the 9th of Sept. 1844, a halo of a somewhat complicated structure was seen by many observers, both at New Haven and at Hartford, Ct. It continued for the space of four hours, commencing about 10 A. M. and ending at 2 P. M.

Its appearance is shown in figure 38., where S represents the sun, A B the ordinary halo of about 45° in di ameter, and C D a circle whose centre was in the zenith while its circumference passed through the sun. Directly north of the zenith, upon the circumference of C D, a parhelion appeared at the intersection of CD with the circles E F and G H, which were both equal in size

to itself.